Although it can be used on any door openings in lightweight structures, this invention and the problems on which it is based are explained in relation to a passenger door in the fuselage shell of an aircraft.
Fuselage shells for aircraft are normally produced in so-called lightweight design from an outer skin which is reinforced on the inside by a two-dimensional structure of stringers running in the longitudinal direction of the aircraft and ribs running transversely to the longitudinal direction of the aircraft as stiffening elements.
In order to provide a door opening that can be sealed by a door in such a fuselage shell, it is necessary to reinforce the edge of the door opening with a suitable door frame which meets a plurality of functional requirements. Firstly the door frame performs the structural function of the outer skin section recessed to form the door opening and of the stiffening elements running on its inside and interrupted by the door opening, so that the stiffness of the fuselage shell is not impaired by the provision of the door opening.
Secondly the door frame generally supports hinge elements for the movable suspension of a door in the door opening and locking elements with which the door can be locked to seal the door opening. For this purpose it is necessary to design the door frame so that it is able to support the weight of the door in any opening condition and absorb the forces generated during opening, closing and locking and/or deflect them to the surrounding fuselage structure. A further requirement of the door frame is that it resists mechanical loads, e.g. impacts when passengers embark and disembark or during loading and unloading of the aircraft, so that no damage occurs to the door frame itself or to the surrounding outer skin.
Aluminum and aluminum alloys have been used for decades as the conventional material for the outer skin, stringers and ribs, as well as for the door frame structures. However, they are being replaced to an increasing extent in aircraft construction by composite fiber materials, in particular by carbon fiber reinforced plastic (CFP), since a lower total weight of the aircraft, and hence lower energy consumption in flight operation, can achieved thereby whilst retaining the same strength and stiffness of the aircraft fuselage. Further advantages of the composite fiber materials relative to aluminum materials are low material fatigue and the absence of corrosion.
However, in order to process composite fiber materials techniques are required which are often distinguished essentially from the techniques used to process aluminum materials. For example, door frame parts can be produced from aluminum by cutting solids aluminum semi-finished products. However, specially shaped laminating devices must be supplied for manufacturing suitable parts from carbon fiber reinforced plastic, in which devices fiber structures can be draped and laminated with epoxy resin in the required target shape. Generally speaking, both the production and the assembly of such CFP components are far more time-consuming and hence more expensive than for corresponding conventional aluminum components. Moreover, complexly shaped components of CFP, in particular, are difficult to repair or cannot be repaired at all, which represents a considerable disadvantage in the case of door frame parts which are subject to continuous risk of damage during operation.
In a conceivable combination of aluminum and CFP components, e.g. a door frame of an aluminum alloy and an outer skin of CFP, the problem arises, on the other hand, that between aluminum and carbon fiber reinforced plastic there is an electrochemical potential which results in corrosion on the part of the aluminum when the two materials come into contact with each other. An expensive insulation by intermediate layers of non-conducting materials is therefore required, which increases the production costs and the total weight of the aircraft.